Previously we reported that the synthesis rate of quadriceps mixed muscle protein (MMP) is increased after resistance training in young and elderly adults. The effect of weight training upon the synthesis rate of individual contractile proteins has not been previously examined. We used a constant intravenous infusion of [13C]leucine to determine the in vivo rate of amino acid incorporation into vastus lateralis MMP, myosin heavy chain (MHC), and actin in 23-30 yr old healthy, sedentary women and men (n=4) before and after 2 wk of resistance exercise. Before training, the fractional synthesis rates of MMP, MHC, and actin were 0.053, 0.042 and 0.077%/h. After two weeks of weight training the synthesis rates of MMP, MHC, and actin increased to 0.101, 0.069, and 0.124 %/h. This represented relative increases of 91% for MMP, 64% for MHC and 61% for actin. These findings suggest that short-term resistance training activates the rate of contractile protein synthesis in young adults. The magnitude of the exercise-induced increase in MMP, MHC and actin protein synthesis rates were different.